The present invention relates to a dual-tone multiple-frequency (DTMF)-signal generating apparatus for use with a telephone communication system, and particularly relates to an apparatus for generating a DTMF signal, corresponding to a key operation of a push button phone, and transmitting it on a standard telephone communication line.
A conventional DTMF signal generating apparatus divides a reference clock signal, generated from a reference oscillating circuit, into each row and column of an actuated key, and produces a DTMF signal corresponding to the actuated key by transforming the frequency-divided signals into sine waveforms with different respective standard frequencies.
A prior art DTMF signal generating apparatus cannot oscillate unless the communication line voltage is above 3.0 V to 3.5 V, and current dissipation is high because of the high frequency, for example, 3.58 MHz, of a frequency clock signal from the reference oscillating circuit.
However, the line voltage of an actual telephone communication line may decrease to about 1.5 V to 2.0 V. With such low voltage, the prior art DTMF signal generating apparatus cannot be operated. Furthermore, in a prior art DTMF signal generating apparatus, the current dissipation is high, and the construction of the frequency divider is complicated. Furthermore, a 3.5 MHz crystal resonator used for the reference oscillating circuit is costly.
To eliminate such drawbacks, lowering the frequency of the reference clock signal has been considered in order to decrease the current dissipation and to operate at a lower voltage.
However, simply lowering the frequency of the reference clock signal makes it difficult to determine the frequency division ratio for dividing the clock signal into standard frequencies for each row and column of the key matrix, and makes the construction of the frequency divider more complicated, thereby making it difficult to product a highly accurate DTMF signal.
Therefore, a DTMF signal generating apparatus which has a simple construction and low cost and which can operate with high stability and lower voltage has been in demand.
In addition to the above, an integrated DTMF signal generating apparatus with CMOS devices has been in strong demand.